Macrophages Refining Lymphatic Sprouts
Macrophages refine (i.e., prune/reshape) developing lymphatic sprouts by recruited phagocytosis of the sprouts during the refinement period , with cell death signals marking the sprouts for clearance . Macrophages are...
Macrophages refine (i.e., prune/reshape) developing lymphatic sprouts by recruited phagocytosis of the sprouts during the refinement period , with cell death signals marking the sprouts for clearance . Macrophages are required for refinement : after refinement, α CSF1R depletion leaves lymphatic vessels “similar to P20” without reduction in total length, sprouts, or perisinusal sprout length, whereas control mice show normal refinement.[:cite[1]{ln=4}], [:cite[1]{ln=5}]. Macrophages phagocytose lymphatic sprout material : using Prox1 GFP (LEC GFP), the paper finds bright GFP clusters near perisinusal sprouts at P20–P21 , and critically these GFP clusters are inside macrophage lysosomes (CD68+) , with 3D analysis confirming engulfment within lysosomes.[:cite[2]{ln=3}], [:cite[2]{ln=4}], [:cite[2]{ln=5}]. This phagocytosis is linked to sprout cell death : the refinement window includes apoptotic lymphatic cells in perisinusal sprouts at P20/21 (cleaved caspase 3+ and TUNEL+), indicating that the sprouts have entered apoptosis prior to refinement.[:cite[3]{ln=1}], [:cite[3]{ln=2}], [:cite[3]{ln=4}]. The study shows that blocking apoptosis with Q VD OPh during refinement increases total lymphatic length and perisinusal sprout length—consistent with refinement failure when sprout death/phagocytosis is blocked.[:cite[4]{ln=4}]. Mechanism of “eat me” recognition via TAM receptors : macrophage phagocytosis for refinement requires TAM receptors ; the study describes MerTK and Axl expression in meningeal macrophages near sprouts and increased expression on GFP material–containing macrophages.[:cite[5]{ln=1}], [:cite[5]{ln=3}]. It also states that PtdSer surface exposure is required to engage TAM receptors.[:cite[5]{ln=4}], and that blocking TAM receptors or inhibiting PtdSer similarly impaired refinement (increased total and perisinusal lymphatic lengths).[:cite[6]{ln=3}], [:cite[6]{ln=4}]. In short: during development, perisinusal lymphatic sprouts enter an apoptosis associated state, and recruited meningeal macrophages phagocytose sprout material (in lysosomes) ; this clearance is required to reduce/reshape sprouts into the refined lymphatic network.[:cite[7]{ln=5}], [:cite[2]{ln=4}], [:cite[4]{ln=4}].